As one of controlling methods for an engine mounted on a vehicle, torque-based (torque demand) control for controlling the intake air amount, fuel injection amount, ignition timing and so forth with reference to the magnitude of a torque demanded to an engine is available. In the torque-based control, the engine is controlled such that, for example, a target value for an engine torque is calculated based on the acceleration opening, engine speed and so forth and a torque having the target value is acquired. Further, in a vehicle in which an external controlling system such as an automatic transmission, an automatic cruise apparatus or a vehicle stabilization apparatus is incorporated, output power demands from the several external controlling systems to the engine are converted into torques and unified by an engine ECU (engine electric controlling apparatus) and then a torque behavior of the engine is comprehensively controlled.
Incidentally, an output power demand to the engine possibly varies not only upon running of the vehicle but also during stopping of the vehicle. For example, if a shift lever operation of an automatic transmission or a starting operation for an external load apparatus such as an air conditioning apparatus or various electric equipments is inputted, then the load acting upon the engine increases in response to the operation. Therefore, it becomes necessary to increase the output power of the engine. Particularly upon idling, the engine speed is lower than that upon traveling of the vehicle and is liable to be influenced by an external load, and the operation condition of the engine is liable to become unstable. Therefore, a technique is known that, as control upon idling, the ignition timing or the intake air amount is changed in response to the magnitude of the external load acting upon the engine to adjust the engine torque to maintain the engine speed at a predetermined idle speed.
For example, Japanese Patent Laid-Open No. 2008-297946 discloses a technique that ignition timing feedback control of controlling the ignition timing and air amount correction control of controlling the intake air amount are used to maintain the engine speed upon idling at an idle speed. In this technique, attention is paid to a difference in torque adjustment width between the ignition timing feedback control and the air amount correction control to carry out one of the controls based on the ignition timing at a point of time at which an external load demand is detected. It is considered that this makes it possible to maintain the engine speed as fixed as possible.
The magnitude of the external load acting upon the engine varies in response to an operation state of the external load apparatus. However, in such a prior art as described in Japanese Patent Laid-Open No. 2008-297946, since an influence of such variation of the external load is not taken into consideration, it is sometimes impossible to cause the actual engine torque to follow up the demand from the external load. Particularly upon idling which is liable to be influenced by an external load, such torque difference appears conspicuously, and there is a subject that it is difficult to assure stability in idle rotation.
Concerning this subject, also it is imaginable to assure the stability by normally advancing the ignition timing irrespective of the magnitude of the external load or by normally increasing the air amount. However, if such a high engine torque as can cover all possible load variations is outputted, then the energy loss increases, resulting in deterioration of the fuel cost.